Benzene is known to be the most valuable aromatic hydrocarbon. It enjoys extensive application in the manufacture of a great variety of chemicals, such as cyclohexane, ethylbenzene, cumene, aniline, which in turn are utilized in the production of plastics, synthetic fibers, rubbers, dye-stuffs and the like.
Heretofore it was customary practice to obtain benzene mainly by catalytic reforming narrow benzine fractions on alumoplatinum catalysts. A substantial amount of benzene is also produced via dealkylation of toluene in the presence of hydrogen (hydrodealkylation) in accordance with the following reaction: EQU C.sub.6 H.sub.5 CH.sub.3 +H.sub.2 .fwdarw.C.sub.6 H.sub.6 +CH.sub.4
Known in the prior art are catalysts for dealkylating alkylbenzenes in the presence of steam, comprising nickel as an active component (U.S. Pat. No. 3,634,532; FRG Pat. No. 2,049,151).
A major disadvantage inherent in the nickel catalysts disclosed therein is their extremely low stability. The interregeneration period of such catalysts is confined to a few hours.
Known in the prior art are catalysts of the platinum group metals deposited on an inert porous carrier, most frequently aluminum oxide (cf. USSR Inventor's Certificate No. 198,310; FRG Pat. No. 1,793,129; U.S. Pat. No. 3,595,932). Use is made of various modifications of aluminum oxide, such as .gamma..sup.-, .eta..sup.-, .delta..sup.-, .theta..sup.-, .alpha.-forms, as well as alumosilicates. The stated catalysts are promoted by alkaline, alkaline earth metals, metals of the Fe subgroup (iron, cobalt, nickel), the V subgroup (vanadium, niobium, tantalum), the Cr subgroup (chromium, molybdenum, tungsten), copper and also by the additions of elements of the lanthanum and actinium groups (U.S. Pat. Nos. 3,436,433; 3,436,434; 3,649,706; 3,649,707; 3,848,014; British Pat. No. 1,313,941; Jap. Appl. No. 74-126,630).
The aforementioned prior art catalysts for steam dealkylation processes look more promising than the nickel-containing contacts insofar as stability is concerned.
A disadvantage of such catalysts lies in the appreciable specific gravity of secondary reactions involved in cleavage of the benzene ring, which tends to impair the selectivity of benzene formation and, as a consequence to decrease the yield of the final product.
Furthermore, an acceptable dealkylation selectivity level of about 90-95 mol.% of that theoretically feasible is attained with the foregoing catalysts only at a relatively low degree of feed conversion (40-60%).
A higher extent of starting feed conversion, e.g. by means of elevating the temperature, inevitably results in further reduction of the process selectivity and, hence, in an increase of costly hydrocarbon feed losses.
Among the platinum group metals, rhodium when applied to aluminum oxide has the highest activity (USSR Inventor's Certificate No. 198,310; FRG Pat. No. 1,793,124). The principal disadvantage of the alumorhodium catalyst is its low selectivity in the reaction of toluene demethylation amounting to 90 mol.% with 60% toluene conversion, which corresponds to 54 mol.% benzene yield.
Alkylaromatic hydrocarbons under the conditions prevalent in the steam dealkylation process, apart from the basic reaction of dealkylating into benzene, are apt to undergo complete decomposition to oxides of carbon and hydrogen, for instance: EQU C.sub.6 H.sub.5 CH.sub.3 +(7+n)H.sub.2 O.fwdarw.CO.sub.2 +(7-n)CO+(II+n)H.sub.2
Ratio of the product yield of a specific dealkylation reaction to the total conversion of hydrocarbon feed characterizes the selectivity of dealkylation, which for the given case of toluene demethylation can be expressed as follows: ##EQU1##
The process selectivity is largely dependent on the properties of catalyst being used.
The catalysts, wherein besides rhodium, additions of other ingredients contributing to the increase of dealkylation selectivity, feature a higher level of selectivity.
Known in the prior art is a catalyst containing the following elements, in wt.%: 0.9% rhodium, 10% chromium oxide, 1% ferric oxide and 2% potassium oxide, the balance being aluminum oxide (U.S. Pat. No. 3,436,433). According to this patent the stated catalyst was employed in the reaction of toluene demethylation with steam.
A major disadvantage of this prior art catalyst is its relatively low selectivity in the specific reaction amounting to 94.9 mol.%. Another disadvantage is the fact that the aforesaid selectivity is achieved at inadequate toluene conversion per run (about 50%).
A higher degree of toluene conversion, as mentioned hereinabove, results in still more drastic reduction of demethylation selectivity, i.e. in decrease of the benzene yield per total amount of toluene reacted.
The promoting additions revealed in the patent do not ensure selectivity of toluene demethylation higher than 95 mol.% at a high degree of feed conversion (above 50%).
Despite the fact that the process of steam dealkylation has a number of obvious advantages, industrial techniques of such a process have not developed, inasmuch as a catalyst featuring the selectivity and stability sufficient for effecting the process on a commercial scale is not available in the present state of the art.